Electromagnetic coils are well known to prior art. A coil winding form, known as a bobbin or former, is created in the desired shape of the coil. The bobbin is wound with wire until a suitable number of turns are produced. The number of turns which are wound onto the bobbin is one factor which determines the ultimate strength of the electromagnetic field that the coil can produce, the strength being approximately proportional to the product of the number of turns of wire times the electrical current flowing in the wire, usually referred to as ampere-turns.
One difficulty with producing coils which have a high number of ampere-turns, and thus a high electromagnetic field strength, is that increasing the current, e.g. the amperage, flowing in the wire also increases the resistive (Joule or Ohm) heating of the coil. This heating represents lost energy in the coil and is a significant component of low efficiency.
An alternative to increasing the current is increasing the number of turns of wire on the bobbin. While this may seem to be an attractive solution, the larger coil takes up more space inside the device in which the coil is mounted. This hampers efforts to keep the device small. One way to combat this problem is by flattening the wire so that it has a square or rectangular cross-section. Such wire, sometimes referred to as ribbon wire, packs more densely than circular wire when it is wrapped onto the bobbin. Unfortunately, when circular wire is flattened in this manner, the resistance of the wire tends to increase due to cold working of the metal. This higher resistance in the wire increases the resistive heating of the coil and lowers its efficiency. As an alternative to flattening ordinary wire, spools of preformed square or rectangular wire can be obtained from wire manufacturers. Preformed wire, since it is square or rectangular to begin with, can be treated (i.e. annealed) at the factory to exhibit less cold working. However, preformed wire has the disadvantage of being more difficult to handle than ordinary circular wire. Preformed wire tends to twist as it is being wound from the spool onto the bobbin, which can lead to uneven coil windings. This twisting can be difficult to prevent, especially for thinner wire whose flat sides may be too small to serve as good guides for controlling the twist.
Another component which influences efficiency is the bobbin around which the coil is wound. Since the bobbin takes up space, it prevents the coil from coming into close proximity to whatever object, i.e. armature, permanent magnet, solenoid shaft, etc., the coil is trying to act against. Since the electromagnetic force produced by the coil on an object is inversely related to the interstitial distance between the coil and the object, increasing the distance reduces the ultimate available force. Thus, achieving a given force over a larger distance requires more ampere-turns than would achieving the same force over a shorter distance. But is mentioned above, increasing the number of ampere-turns may have undesirable consequences.
In connection with interferometer spectrometers, electromagnetic coils are used in linear motors for producing high-accuracy linear translation of the movable mirror of such inferometers. The coil is typically attached to a support element of the movable mirror, and permanent magnets are attached to the interferometer housing in close proximity to the coil. Applying a time varying electrical signal to the coil, such as a ramp function, allows smooth and accurate linear translation of the movable mirror. This arrangement is known to provide extremely good linear translation of the movable mirror, which is important to the successful operation of the interferometer. An example of such an arrangement is shown in U.S. Pat. No. 4,693,603, issued Sept. 15, 1987 to Auth, which is incorporated herein by reference.
With respect to linear motors for use in interferometers, it has been observed that, in certain instances of large mirror displacement, the bobbin tends to collide with the sides of the bore in which the bobbin moves. One reason for this is that the moving mirror assembly, including the bobbin, is commonly mounted on a parallelogram support system which provides longitudinal translation of the moving mirror by swinging the mirror back and forth in an arc. With an accurate parallelogram support system, the mirror can be translated through a relatively large longitudinal range of motion without significantly disturbing its alignment. Of course, as it swings back and forth in an arc, the mirror support assembly also moves up and down, producing a lateral displacement of the bobbin within the bore. As the mirror swings through larger longitudinal strokes, which it may do in order to obtain increased spectral resolution from the interferometer, the lateral displacement of the bobbin also gets larger, until eventually the bobbin collides with the sides of the bore. The collisions can sometimes cause the bobbin to jam in the bore, and can occasionally damage the interferometer.
One way to reduce the chance of the bobbin colliding with the bore is mentioned in the above referenced U.S. Patent. By making the lateral cross-section of the bobbin and the bore rectangular, the sides of the bobbin and the bore can be made parallel to each other and orthogonal to the axis of the swinging mirror support, so that the distance between the bobbin and bore does not change much as the bobbin moves back and forth in its stroke. The bobbin is therefore less prone to colliding with the sides of the bore.
However, it has also been observed that when a coil is wound on a rectangular bobbin, the electromagnetic field produced by the coil may not be uniform. In particular, when it is used to drive the moving mirror in an interferometer, the mirror often exhibits some nonlinearities in its back and forth travel. Although this phenomenon is not well understood, it is believed to be due in part to fringing effects which occur in the magnetic field lines at the corners of the coil. Circular coils, being symmetrical, do not have this problem.
An additional problem with the rectangular bobbins is that they are more difficult to wind than are circular ones. With rectangular bobbins the tension of the wire changes as the different corners and faces of the bobbin are encountered, making it more difficult to keep the windings uniform. Another problem is that the windings are often bent very severely around the corners of the bobbin, which can cause the wire to work-harden, or even break, at those points. Work-hardening is undesirable because it increases the electrical resistance of the wire, which increases its heat dissipation and reduces its efficiency. Circular bobbins, again, do not encounter these problems.
Another disadvantage of rectangular bobbins is that rectangular coils tend to be less efficient than circular coils. When used in the manner suggested by the above U.S. Patent, the only portion of the coils that does any work in the portion that is parallel to the permanent magnets in the bore. The portion of the coils which is orthogonal to the permanent magnets does not do any work, so the current which flows through this portion of the coils is wasted. One solution to this problem is to increase the aspect ratio (the ratio of the long side to the short side) of the bobbin so that most of the winding is parallel to the magnets. Then, only a small portion of the current is wasted. Unfortunately, increasing the aspect ratio makes the windings even more difficult to produce, because of the severe changes in tension, and also more likely to have work hardening or wire breakage at the corners. Again, a circular coil would avoid these problems because the permanent magnets could be positioned all around its periphery.